How to clean out fuel tank and flush fuel system - BEFORE you try to start the car
I've mucked out 2 of these tanks, and assisted on a third. With both of my cars they had sat for a dozen or more years with fuel in the tank.
DO NOT even tickle that starter key until you clean out and flush the fuel system, and check oil etc. If contaminants in the fuel system get into your fuel distributor, you have to replace it or send it in for a rebuild - this is not a casual warning. The B12 used in this process does not belong in the fuel distributor.
Go to the auto parts store and find a $12 electric fuel pump, some hose and a gas can if there is still gas in the tank, rig it all up to pump out the gas as much as you can. More likely there is some mix of gas and "tar" that you will need to clean out.
also get some clear plastic cheap fuel filters with paper media, 2 or 3.
Buy a gallon of B12 chemtool if you can get it. Seafoam may work too.
Get a tyvek suit and chemical resistant gloves.
Get a small can of good gas.
Buy a new pump and fuel filter and accumulator (and Herveys baffle if you like it - its fine and I like it better but some people don't - your call - check back if you want to know the pros and cons. buy a new 12" power harness, upper and lower fuel boot, pickup hose and filter, sealing gasket for fuel sender, and the fuel lines from the pump to the metal pipes.
Get 2 plastic tubs, half gallon to gallon. A 1/4" bolt 2" long with large fender washers and maybe a couple nitrile rubber washers
Get a full set of copper washers from your favorite vendor for the fuel system.
Cleaning the tank:
Remove access covers in front trunk.
Remove fuel level sender and fuel pump and baffle assembly. Save the oem baffle even if you decide to go aftermarket.
Leave the fuel lines from the top of the pump connected to the car body for now, note which one was connected to the top of the pump/
Pump out any gas using your cheap-o fuel pump
Suit up in tyvek, and clean out the tank - you will get tar everywhere and the suit will be trashed when you are done so have someone on hand to fetch stuff and be prepared to do the whole tank cleaning job at one go.
Use a bit of B12 and rags or heavy duty paper towels to muck out the tank, you have to work at it to get at the whole tank, but it can be done unless you have 21" biceps (is your wife a REALLY good sport and are you ready to burn ALL your Delorean good will on this one? Better to do this one yourself.)
There is a grounding wire of some sort inside the tank, it stays - don't mess with it.
You can finish with a can of carb cleaner, spray on a paper towell, when the the towels come out clean, the tank is as fresh as new.
Before you reassemble I also suggest a flush of the fuel system a follows:
Rig up your cheapo pump in the front boot so it can draw B12 out of one of your tubs, through a plastic filter, so you can pump it down the supply fuel line that was tied to the oem pump. Let the return line sit in the tub.
Now, don't flush the fuel distributor, instead go to back to the engine area and locate the fuel feed and return lines at the distributor and disconnect them. These are large banjo fittings on the ends of the hoses, don't think about trying to disconnect them. Instead put a fender washer and rubber gaskets onto your 1/4" bolt and slide on one bango, the the other, then another rubber washer and fender washer, finally the nut. The fuel will flow, but it also will leak. Set this up so it cannot drip or run down the lines and drip, and get the connection into the second small tub. keep an eye on it during the flushing process, especially the first time you turn on the cheap pump.
Once you start flushing, be prepared to finish - don't stop half way through - B12 is too strong to leave in the system for any extended period of time.
Pour a quart or so of B12 into the front tub, and start the pump. Eventually black fluid and clumps will come out of the return line. Run it for a few minutes, then change out to fresh fluid, a fresh paper filter, and repeat. Do this several times, until the fluid is generally clear, if not tinted. As soon as the B12 is clean, stop flushing with B12 and flush with clean gasoline immediately, do not leave B12 in the system for an extended period of time.
Clean your fuel level sender.
Re-attach the fuel lines at the distributor (use fresh copper washers), now replace the two fuel lines from the fuel tank to the hard pipes near the tank, and reassemble. If you are using the oem baffle and pick up, look at the mod of putting a spring inside the line so it won't collapse. Install the new (12-18") fuel pump wires) and the fuel sender unit.
The fuel system can now be tested temporarily - but not run for long periods. You also need to replace the fuel filter and the accumulator (that job sucks), have your injectors cleaned, and replace all the copper washers. But, its ok to test it.
Good luck. Track me down if you need clarification.